MSL 3.2.1 is based on version 3.2 of the Modelica (language) specification and that's what SystemModeler supports. Now if you read about Modelica_Synchronous:
Free library to precisely define and synchronize sampled data systems with different sampling rates. It provides convenient to use blocks to utilize the new synchronous language elements introduced in Modelica 3.3.
So it requires Modelica 3.3 language features and that isn't supported by the current version of SystemModeler.
Otto
